By measuring the directional energy distribution of radiation reflected from a sample surface, scatterometers provide a simple and noncontact monitoring technique to determine the surface and sub-surface microstructure of the sample. For example, if the sample surface is perfectly mat then the reflected radiation is diffuse, i.e., equal in all directions, while if the sample surface is not perfectly mat then the reflected radiation is specular, i.e., more concentrated in certain directions. This technique is useful in many areas of technology, such as fabrication of microelectronics materials, fabrication of optoelectronics materials and manufacturing of computer disks, where it is important to determine whether there is any defect in a sample material. Moreover, since this technique is noncontact, it allows measurements to be made in-situ to assist the quality control of manufacturing processes used in these areas of technology.
An existing problem in the field of scatterometry is the lack of optical systems which can provide accurate measurements of both diffuse and specular reflectance, control of the angle of incidence of the radiation beam directed upon the sample surface, and simultaneous measurements of all radiation reflected from the sample surface to represent the directional reflectance distribution as a two-dimensional image. Certain prior art scatterometers have some, but not all, of the above desired features.
For example, a prior art scatterometer, described in U.S. Pat. No. 5,241,369, employs a screen positioned to receive and display a pattern representative of light specularly reflected and scattered from an illuminated sample and a camera to record the pattern displayed on the screen. The use of the screen results in a re-illumination of the sample by reflections from the screen, corrupting the measurements. Consequently, accurate measurements of reflectance distribution are not possible with this prior art scatterometer.
Another prior art scatterometer, described in U.S. Pat. No. 4,988,205, comprising a primary mirror, a secondary mirror, a radiation source and a radiation detector, provides accurate measurements of only total diffuse reflectance. This prior art scatterometer does not provide measurements of directional reflectance. Moreover, the angle of incidence of the radiation beam directed upon the sample surface is fixed and cannot be regulated during the operation of this prior art scatterometer.